Characterization of chloride transport in the unsaturated zone near salted road

Abstract

The application of road salts as deicing agents contributes to the anthropogenic loading of chloride (Cl−) on the environment. Using a 2D solute model, the potential of the unsaturated zone to serve as a reservoir and the mechanisms controlling the movement of Cl− were examined. Physical properties and initial pore-water Cl− concentrations were derived from an array of soil borings. Initial pore-water concentrations show the presence of a Cl− “slug” approximately 0.75 m below the surface. Simulations show that within the unsaturated zone, Cl− transport is predominantly vertical, driven by molecular diffusion. After a 1-year simulation, a Cl− slug similar to the background observation was noted. While Cl− is retained in the unsaturated zone, a net loss of Cl− from the unsaturated zone was simulated for the first 10 years. In year 11, an equilibrium between the Cl− input and output is achieved. The presence of Cl− in the unsaturated zone becomes a long-term source of chloride to the groundwater, which eventually discharges into the surface water. Historically, surface water chemistry data support the continual discharge of chloride to the surface water in the area, further supporting the hypothesis that the unsaturated zone serves as a Cl− reservoir.